110 THE VERTEBRATE SKELETON. 



pairs in the upper jaw, one in the lower. These structures 

 persist throughout life and grow continuously. The upper 

 tooth structures are attached respectively to the ethmoid or 

 vomerine region of the skull, and to the palato-pterygoids. The 

 vomerine teeth are small, while those attached to the mandible 

 and the palato-pterygoid region are large and bear several 

 roughened ridges adapted for grinding food. The teeth of 

 the two opposite sides of the jaw meet in a median symphysis. 

 The teeth of Chimaera are more adapted for cutting, those of 

 Callorhynchus for crushing. Many extinct forms are known, 

 some of whose teeth are intermediate in structure between 

 those of Chimaera and Callorhynchus. 



The teeth of GANOIDS are also extremely variable. Among 

 living forms, the Holostei are more richly provided with teeth 

 than are any other fishes, as they may occur on the pre- 

 maxillae, maxillae, palatines, pterygoids, parasphenoid, voiners, 

 dentaries, and splenials. Among the Chondrostei, on the other 

 hand, the adult Acipenseridae are toothless ; small teeth how- 

 ever occur in the larval sturgeon, and in Polyodon many 

 small teeth are found attached merely to the mucous membrane 

 of the jaws. Many fossil Ganoids have numerous flattened or 

 knob-like teeth, borne on the maxillae, palatines, vomers and 

 dentaries. Others have a distinctly heterodont dentition. 

 Thus in Lepidotus the prernaxillae bear chisel-like teeth, while 

 knob-like teeth occur on the maxillae, palatines and vomers. 

 In Rhizodus all the teeth are pointed, but while the majority 

 are small a few very large ones are interspersed. 



In TELEOSTEANS, too, the teeth are eminently variable both 

 in form and mode of arrangement. They may be simple and 

 isolated, or compound, and may be borne on almost any of the 

 bones bounding the mouth cavity, and also as in the Pike, on 

 the hyoid and branchial arches. The splenial however never 

 bears teeth and the pterygoid and parasphenoid only rarely, 

 thus differing from the arrangement in the Holostei. 



The isolated teeth are generally conical in form and are 



